[FIX] base: res.config default value on selection fields
[FIX] base: res.config default value on selection fields

Since 3d8f7c2f, res.config support selection fields to set groups and install
modules using a selection field instead of boolean.
While the `module_*` fields are correctly casted from boolean to integers, it
was not the case with `group_*` fields.

This commit forces the conversion to get the expected field format value.

Closes #10373

sysadminmatmoz/OCB,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,Elico-Corp/odoo_OCB,microcom/odoo,hip-odoo/odoo,microcom/odoo,sysadminmatmoz/OCB,dfang/odoo,sysadminmatmoz/OCB,storm-computers/odoo,ygol/odoo,hip-odoo/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,dfang/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,storm-computers/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,laslabs/odoo,ygol/odoo,ygol/odoo,microcom/odoo,microcom/odoo,storm-computers/odoo,ygol/odoo,microcom/odoo,bplancher/odoo,hip-odoo/odoo,bplancher/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,microcom/odoo,laslabs/odoo,hip-odoo/odoo,laslabs/odoo,storm-computers/odoo,hip-odoo/odoo,dfang/odoo,storm-computers/odoo,laslabs/odoo,sysadminmatmoz/OCB,dfang/odoo,sysadminmatmoz/OCB,laslabs/odoo,ygol/odoo,dfang/odoo,storm-computers/odoo,dfang/odoo